Genetic variants in progranulin upstream open reading frames increase downstream protein expression

Abstract

Premature termination codon (PTC) mutations in the granulin gene (GRN) lead to loss-of-function (LOF) of the progranulin protein (PGRN), causing frontotemporal lobar degeneration (FTLD) by haploinsufficiency. GRN expression is regulated at multiple levels, including the 5′ untranslated region (UTR). The main 5′ UTR of GRN and an alternative 5′ UTR, contain upstream open reading frames (uORFs). These mRNA elements generally act as cis-repressors of translation. Disruption of each uORF of the alternative 5′ UTR, increases protein expression with the 2 ATG-initiated uORFs being capable of initiating translation. We performed targeted sequencing of the uORF regions in a Flanders-Belgian cohort of patients with frontotemporal dementia (FTD) and identified 2 genetic variants, one in each 5′ UTR. Both variants increase downstream protein levels, with the main 5′ UTR variant rs76783532 causing a significant 1.5-fold increase in protein expression. We observed that the presence of functional uORFs in the alternative 5′ UTR act as potential regulators of PGRN expression and demonstrate that genetic variation within GRN uORFs can alter their function.